1. Field of the Invention
This invention relates to a process of manufacturing shaped elements from synthetic thermoplastics, particular polyolefins, such as polyethylene and polypropylene, wherein the plastic material is heated to a temperature above its softening temperature and is shaped in that state to form the shaped element and is subsequently cooled to a temperature below its softening temperature.
2. Description of the Prior Art
Numerous plastic parts which consist of a synthetic thermoplastic material and are made from the hot material by injection molding, extruding, compression molding or a different shaping operation tend to deform in the course of time and/or at temperatures above temperature limits which are far below the softening temperatures and under such conditions tend particularly to shrink, i.e., to decrease their outside dimensions relative to the dimensions of the element as made. For instance, tubular elements which are made of black polyethylene will after a certain time of outdoor storage exhibit a certain shrinkage, which maybe of an order of one or a few millimeters or percent of the initial diameter. Although the conventional manufacturing temperatures or softening temperatures of sockets lie in the range from 120.degree. to 200.degree. C. or for strict shaping operations are about 110.degree. C. and black tubular elements are not heated to temperatures in excess of about 70.degree. C. by solar irradiation, such shrinkages do actually occur in practice. This is undesired as regards the flow rate which can be achieved and is most inconvenient for the making of joints between tubular elements because conventional joints between sockets and spigots cannot be made satisfactorily if certain shrinkages have occurred and because sockets with which such tubular elements have integrally been formed will have after a shrinkage an inside diameter which is smaller than the outside diameter of the tubular elements which are to be joined. Usual manufacturing tolerances for the inside diameters of sockets are .+-.0.5 mm. Said tolerances are greatly exceeded by the above-mentioned shrinkages. Similar disadvantages will occur in connection with a large number of plastic parts, such as pipe fittings, which are made from polyolefins by hot-forming.
For making shrinkage-proof sockets it has previously been necessary to use expensive manufacturing processes. In one of said known processes the sockets are separately made in an injection molding machine and are subsequently welded by means of a separate device to an extruded tubular element. That process is highly expensive and results in a visible seam weld at the joint so that the risk of leakage is increased by an oxidation of the material and the socket may even become detached at the welded joint and because the socket made by injection molding may shrink, e.g., under solar irradiation, if certain materials have been used, so that an adequate safety will not be achieved. The same disadvantages are encountered in another known process, in which sockets are molded in a separate machine by means of an injection molding device onto the end of a tubular element. That practice will involve an even higher technical expenditure owing to the use of expensive machines and the tubular element and the socket will not constitute a homogeneous body.
The shrinking of certain plastics, particularly polyolefins, may be explained as follows:
Polyolefins have a crystalline structure and occur only in two physical states, namely, the solid and liquid states. When the plastic is heated, a part of the original molecules remain solid and the other part are liquefied so that a plastically deformable element is obtained, which can be shaped and deformed. The presence of those molecules which are not liquefied but remain solid gives rise to the so-called memory effect, which means that a shaped element made from a blank tends to return to its initial shape as soon as the molecules which have previously been liquefied and tend to maintain the shaped element in its current shape are heated or undergo a long-time shrinkage so that they permit the corresponding reverse deformation. It will be understood that the memory effect will also be influenced by the amount and nature of any possible fillers. The memory effect will also be observed in tubular elements made by extruding and will be particularly pronounced in expanded sockets and may be regarded as the main cause of the shrinking of tubular elements and sockets. Similar causes result in a shrinkage of other plastic parts made from the above-mentioned materials.